Continuously variable transmissions (CVTs) utilizing a hydrostatic power unit are well known. A variety of work machines utilize this type of transmission for industries such as agriculture, earth moving, construction, forestry, and mining. During operation, fluid displacement of the hydrostatic, power unit is varied to continuously change the output to input ratio of the transmission (i.e., the transmission ratio). This is accomplished by adjusting the angle of a swash plate of a variable displacement fluid pump or motor of the hydrostatic unit. One example of a CVT utilizing a hydrostatic power unit is described in U.S. Pat. No. 7,063,638 to Weeramantry, which is hereby incorporated by reference in its entirety herein for all purposes.
In order to control the speed of a work vehicle, it is known to have a CVT operably connected to a clutch or “inching” pedal. For example, an operator may be permitted to slow down or stop a work vehicle by simply depressing the inching pedal. When the pedal is depressed, the pressure within an engaged clutch of the transmission may be reduced, which results in slippage of the clutch and less torque being transferred through the transmission. However, while the inching pedal provides an effective means of slowing down a work vehicle, its use can be problematic when the vehicle is operating on an inclined surface (e.g., a hill). In such instances, when the inching pedal is depressed and the work vehicle decelerates, the vehicle may actually reverse directions and begin to roll down the inclined surface. To control such “rollback” of the vehicle, conventional control methods typically require that a shuttle shift be performed as the vehicle reverses directions by swapping directional clutches within the transmission, such as by swapping from the forward directional clutch to the reverse directional clutch). Unfortunately, these control methods make it difficult to control the motion of the vehicle as it reverses directions. For example, the change in torque required to perform the shuttle shift often results in jerky motion that can be felt by the operator.
Accordingly, an improved system and method for controlling rollback of a work vehicle would be welcomed in the technology.